In digital signal processing apparatuses, signal processing speed has been speeding up, and ensuring signal quality and reducing radiation noise are both desired. Especially, in outside wiring of digital devices for transmitting signals between the devices, the transmission distances are often long. In such a case, it is difficult to install a metal housing that serves as a stable ground in the vicinity of a signal wire. Accordingly, the achievement of both ensuring signal quality and reducing radiation noise is highly required. Further, even in wiring inside the digital devices, it is difficult to reduce radiation noise by using its housing since the recent housing is often non-conductive. Further, when a signal is transmitted and received between a digital device and a movable member, for example, in a digital multifunctional device, stable transmission between an image reading device and an image signal processing device is difficult since the image reading device is moving and the positional relationship of the cable to the metal housing is not fixed. To solve the problems, a shielded cable is often used in the wiring outside the digital devices, in the wiring within the digital devices that require high-speed signal transmission, or in the signal communication with movable members. By using the shielded cable, superimposition of radiation noise from outside on a communication signal flowing in a signal wire provided inside the shielded cable can be prevented. Further, reduction of radiation noise to the outside can be expected. Moreover, the coupling between the shielding layer and the signal wire is strong, and the shield member serves as a stable signal ground. Accordingly, increase of the signal quality can be expected.
Conventionally, a shielded cable is known that has a shielding layer formed by helically winding a tape-shaped shield member of a metal layer such as aluminum or copper and an insulating layer such as a plastic film laminated and integrated onto a signal wire to cover the signal wire. The shielded cable formed by helically winding the tape-shaped shield member has the simple structure. Accordingly, as compared to a shielded cable formed by braiding a wire, the productivity of the shield cable is higher. Moreover, the tape-shaped shield member can be tightly wound onto the signal wire. Accordingly, the shield member serves as a stable signal ground.
However, it is known that the radiation noise reduction effect of the shielded cable formed by winding the tape-shaped shield member is lower than that of the braided shielded cable. The reasons of the low radiation noise reduction effect of the shielded cable formed by winding the tape-shaped shield member are as follows. In order to obtain a sufficient radiation noise reduction effect in a frequency band of 30 MHz or more, the thickness of the metal layer covering the signal wire should be about several tens of micrometers. However, in a step for winding the tape-shaped shield member at the time of manufacturing the cable, tension is applied to the shield member. In order to prevent the metal layer from being fractured by the tension, it is necessary to use a shield member of a plastic film of, for example, polyethylene terephthalate (PET) that has large tensile stress and is laminated by a metal layer such as aluminum or copper. In this structure, in an overlapping portion of the shield member, the insulating layer is sandwiched between the metal layers. Accordingly, the metal layers of the wound shield member are not in contact with each other. Thus, the conduction is generated only in the winding direction of the tape-shaped shield member. More specifically, in a high-frequency band, the insulating layer sandwiched between the metal layers in the overlapping portion of the shield member functions as a shield opening in a direction preventing the flow of a return current flowing in the shield member (shielding layer), which reduces the radiation noise suppression effect. To solve the problem, Japanese Patent Application Laid-Open No. 2002-75076 proposes a shielded cable formed of double shielding layers inside and outside by doubly winding the above-described tape-shaped shield member such that the metal layers face each other and come in contact with each other.
However, since the above-described known shielded cable has the double shield structure formed by doubly winding the tape-shaped shield member such that the metal layers face each other and come in contact with each other, there are the following problems. Since the shield member is to be doubly wound, the amount used for the tape-shaped shield member is about twice as much as the amount used for a single shield member, the cost is increased. Moreover, the diameter of the cable is larger than that of the single cable. Accordingly, wiring of the cable in a narrow space is difficult. Further, the flexibility of the cable is low, and wiring performance also becomes low.